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Will the 2440 Diaphragm Work with the 2441 Motor?

While this is my first post, I've been a long-time reader. I have learned much from the forum members and am grateful for the contributions and sharing, especially of some of the 'old timers' (intended as a compliment).

Question - Will 2440 phragms work with the 2441 driver?

There are numerous discussions in the archives about retrofitting 2441 phragms in 2440 motors, but I'm interested in comparing my 2441s with 2440s. In lieu of buying a second pair of drivers (2440s), what are the drawbacks/issues, if any, going the other way around? In other words, substituting 2440 phragms for my 2441s?

It seems the motivation for those plugging 2441 phragms in 2440 bodies was to obtain extended HF. I gather this was the main benefit of the diamond surrounds on the newer phragms. Since I'm using tweeters > 12k (Fostex T925), I'm less concerned with HF extension, and more interested in SQ results <8k or so. I'm interested in gaining first-hand listening experience with the mylar surround as a potential enhancement for my midrange SQ.

So, to obtain first-hand listening experience of the '2440 sound', will a straight-forward diaphragm substitution in my 2441 bodies be representative?

There more to this story...

Similar, sort of....

Bruce Edgar reported the 2441 had at least 3 different versions of the phase plug over the production life cycle, as if the engineers continually refined the driver. Additionally, the two drivers differ in their flux density. Whether the difference is real or the result of using different measurement techniques is unclear.

Another user (can't remember name) reported differences in gap size between the drivers, specifically that the 2441 gap was smaller compared to his 2440. He discovered this when substituting 2441 phragms in his 2440 bodies. Again, unclear if this was driver specific or difference in design specs between the drivers.

So there are real differences between the motors. Whether the differences are relevant or not is open to discussion. I'm just wondering if I can plug the 2440 phragm in my 2441s and gain a representative example of the 2440 sound.

refinements with age

Over the years the 375/2440/2441 have had minor changes in the way the phase plug was centered / machined but not the geometry nor that of the gap UNLESS you look at pre-pro division 375 specs ( 1970 ).

Only when JBL went to the ferrite's ( 2445 ) did it change from bakelite to aluminum half way thru production and then a new geometry with the 2446/2447 and further with the newest, 2452.

And the flux densities change with each printing of the data sheets so take those "issues" with a grain of salt. Welcome to JBL's famous typo artifacts and it's funny to notice that these differences get spouted as fact by the tweako crowd without actually measuring.

If using these older alnico drivers I would have them recharged anyways.

All the diaphrams are interchangeable within the 2" family except the 2482/2485 phenolics. Period. relax.

Any change in "sound" between your drivers will be at the very top-end of the operating range and (A) no horn will faithfully disperse it and (B) you should have a tweeter in around 8-9K anyways.

I dont know why you would want to do that,,,The diamond pattern outer surround of the 2441 is more extended on the top end...And you could xover higher to a tweeter..I think its a waste of time and money,, You could do better going to a TrueExtent Be diaphram..Of course the cost will be much higher.

Most people with 2440 drivers will fit a 2441 diaphragm because it is the better one. If you already have a 2441 driver with 2441 diaphragm, then you are already in the right direction. There are two "legit" diaphragms that you could explore (many graphs have been posted on this forum). One is the expensive Truextent Be diaphragm and the othe one is the very affordable Radian. But as was mentionned, because the diaphragm and driver can reach up to a gazillion Hertz, it dosn't mean the horn can follow. Therefore, if your goal is to crossover to 12Khz, then you'll also need a horn that goes up there too. Most will fall dramatically around 10Khz.

***But as was mentionned, because the diaphragm and driver can reach up to a gazillion Hertz, it dosn't mean the horn can follow. Therefore, if your goal is to crossover to 12Khz, then you'll also need a horn that goes up there too

good point. And my goal is NOT more extended HF bandwidth. Instead, I want very good sound in the linear operating range of the 2441 driver. My understanding is the 2441 HF advantage over the 2440 is gained by resonances due to the stiff diamond surround. I'm inclined to LIMIT BW on the 2441, crossing lower, if the 2440 phragm sounds better in the lower range. I'm fine with reaching a little (just a little) lower with my tweeter, say 8 - 9k or so, if this results in better sound from the mids. I also have a pair of 1" Coral M100s I could cut in between the 2441 and Fostex tweet. In effect slicing to narrower BWs for each driver, to operate each driver more linearly. The other side of the coin is added crossover points.

Here is a comparison JBL did of the 2441, 2440, and TAD TD-4001 all on the JBL 2350 radial horn. It clearly shows the relative difference between the 2440 and 2441 in the midband as well as their respective break-ups.

Here is a comparison JBL did of the 2441, 2440, and TAD TD-4001 all on the JBL 2350 radial horn. It clearly shows the relative difference between the 2440 and 2441 in the midband as well as their respective break-ups.

Widget

Thanks Mr. Widget - never saw the TAD data . . . very impressive. Far more so than True Extent. But, as someone said above, you gotta have a matching horn - just throwing this stuff together accomplishes little if anything. I'm afraid "gear heads" too often miss the point - it's a SYSTEM.

Over the years the 375/2440/2441 have had minor changes in the way the phase plug was centered / machined but not the geometry nor that of the gap UNLESS you look at pre-pro division 375 specs ( 1970 ).

Only when JBL went to the ferrite's ( 2445 ) did it change from bakelite to aluminum half way thru production and then a new geometry with the 2446/2447 and further with the newest, 2452.

And the flux densities change with each printing of the data sheets so take those "issues" with a grain of salt. Welcome to JBL's famous typo artifacts and it's funny to notice that these differences get spouted as fact by the tweako crowd without actually measuring.

If using these older alnico drivers I would have them recharged anyways.

Agree on the last sentence. I just re-gaussed a large brace of drivers, among them 2440 and 2441 types.
Most of them had about 1.6 Tesla residual B, no matter whether 2440 or 2441 core. A 2440 is rated at 2.05 T.

I wish i had taken more photos, because i noticed two things :

a) the 2441 cannot be re-gaussed to more than 1.82 T (the Teslameter used had a traceable professional calibration). This is a gap maximum (see below). Data sheet value for 2441 is 1.8 T, not 2.05 T as in 2440 specs.
You are right about JBL's occasional earlier data sheet confusion. However, it could be that sometimes, this resulted from inappropriate quality T&M gear or sample-to-sample variation, not always editorial mishaps. While 2.05 T could not be verified that day, the 2440s did go up to 2 T, most hovered in the range between 1.91 and 1.97 Tesla. Since the coil is underhung, a 2440 has almost 10% more B and hence B*L using the same fram. Mind you, DCR value varies by more than 10% and assuming constant wire cross-sectional area, DCR neatly translates to L and hence BL ...

The variation between core samples was lower than the variation at different parts of the gap circle in one and the same unit. The gaps simply are not perfectly centered. Plus, some gaps are far from perfect circles, i.e. the outer ring (pole plate) encircling the phase plug part of magnetic return are not always machined very well. On 2441, this problem is less apparent. Another difference: all 2441 that i had on the day appeared to have the gap widened at the point where the voice coil is connected in the fram. Just like in 2445/46/47/50/51. I don't know whether this is true for early 2441 also, the ones i had carried ser# in the range between 24000 and 35k. If anything, then 2441 gap appears wider than 2440, but i had no gauge to document it. Both drives leave less room than their successors. The ferrite (and neo) motors offer much more gap width and likely compensate for the losses by throwing in more magnetic material. Someone here once wrote this was done in order to increase reliability in "hot" situations when the coil expands.

b) the units had around 5 distinctive phase plugs, the material going from a matte "dry looking" light grey right into a shiny "juicy" looking deep black. The spacers between phase plug parts were partly white plastic discs, sometimes rectangular elements of what appeared to be the same material as the phase plug proper. On some, slit width did not appear to be constant by design, esp on the 3rd one, counting from the gap.

Measurements taken showed that all drivers' response depended more on the particular sample of fram than the above-mentioned B or 'plug variations.
Given same fram, small differences were noticeable but rather academic. So the fram makes the music after all and the thread went the right direction (until i came along ;-).